Many current integrated circuit technologies, especially those employing sub-half-micron minimum feature sizes, use titanium silicide (TiSi.sub.x, where x is typically 2) contacts to silicon in order to assure low contact resistance.
FIGS. 1 through 7 describe conventional steps used to create metal-to-silicon contacts. Beginning with FIG. 1, an oxide layer 110 is conventionally grown on the surface of a silicon body 100 by subjecting exposed portions of silicon body 100 to an oxidizing environment. Referring next to FIG. 2, a titanium silicide layer 120 is typically formed by opening a window in the oxide layer 110 to expose an area of silicon body 100 and depositing a titanium layer 115 over the exposed surfaces of silicon body 100 and oxide layer 110.
Silicon body 100, oxide layer 110, and titanium layer 115 are annealed at high temperature, typically with rapid thermal anneal (RTA) in the range of 600.degree. C. to 900.degree. C. Titanium layer 115 does not react with oxide layer 110, but reacts with the exposed surface of silicon body 100 to form titanium silicide layer 120, as shown in FIG. 3. The unreacted portions of titanium layer 115 are then removed, typically using a wet chemical etch.
FIG. 4 shows a conventional titanium silicide contact. To form this contact from the structure of FIG. 3, a dielectric layer 130, which in one embodiment is silicon dioxide, is deposited over the surfaces of oxide layer 110 and titanium silicide layer 120. A contact hole 140 is then etched in dielectric layer 130 to uncover titanium silicide layer 120. Any of a number of conventional wet and dry etching techniques may be used to create contact hole 140. However, wet etching techniques are not effective for feature sizes of less than approximately 3 .mu.m.
As shown in FIG. 5, to the structure of FIG. 4 is added a titanium layer 150 using conventional titanium deposition processes, such as physical or chemical vapor deposition. Native, non-conductive metal oxides that may have formed on the surface of titanium silicide layer 120 are consumed by the titanium (i.e., some portion of the titanium oxidizes, and the titanium oxide disburses throughout titanium layer 150). The consumption of native oxide results in a good metallurgical connection between titanium layer 150 and titanium silicide layer 120. Titanium layer 150 therefore provides low electrical contact resistance to underlying titanium silicide layer 120.
Referring to FIG. 6, a titanium nitride layer 160 is deposited over titanium layer 150 using conventional titanium nitride deposition techniques, such as physical or chemical vapor deposition. The deposition of titanium nitride layer 160 is followed by a conventional annealing process, such as rapid thermal annealing (RTA), typically at temperatures ranging from 500.degree. C. to 700.degree. C. Titanium nitride layer 160 provides a protective barrier for titanium layer 150 during the chemical vapor deposition of tungsten contacts and acts as an adhesion promoter for tungsten contact formation.
FIG. 7 shows a conventional metal-to-silicon contact formed after a tungsten layer 170 has been deposited, using conventional deposition techniques, over the surface of titanium nitride layer 160.
There are numerous problems associated with the conventional process of FIGS. 1 through 7. For example, in the tungsten deposition step of FIG. 7, tungsten hexafluoride gas (WF.sub.6) is introduced onto the surface of titanium nitride layer 160. Unfortunately, tungsten hexafluoride reacts violently with elemental titanium, such as that of titanium layer 150. As a result, any flaw in the protective titanium nitride layer 150 that allows tungsten hexafluoride to contact the underlying titanium layer 150 may result in a defect. This source of defects is a major concern, particularly when employing sub-half-micron integrated circuit technologies. Moreover, the deposition of titanium layer 150 is expensive and wastes valuable processing time.
For the foregoing reasons, there is a need for a simpler, more reliable process that avoids the unnecessary formation of defects caused by the interaction of tungsten hexafluoride gas with elemental titanium.